Euthanasia is the act of deliberately ending a person's life to relieve suffering.
For example, it could be considered euthanasia if a doctor deliberately gave a patient with a terminal illness drugs they do not otherwise need for their comfort, such as an overdose of sedatives or muscle relaxant, with the sole aim of ending their life.
Assisted suicide is the act of deliberately assisting or encouraging another person to kill themselves. If a relative of a person with a terminal illness obtained strong sedatives, knowing that the person intended to use the sedatives to kill themselves, they may be considered to be assisting suicide.